The network consists of an anonymous holding company owned by the Office of Personal Transportation (OPT). This company owns, through a morass of untraceable interlocking companies and holders, a string of sector-wide companies that ostensibly has no connection to each other, such as Core Carriers in Core, Khugishu Bilanidin in Vland, Corridor Caravels in Corridor, Bright Star Lines in Deneb, Medurma Main StarLines in the Dagudashaag Sector, etc..
Although the companies engage in real trade, their primary purpose is to provide a covert transport and courier system for the Imperial government. Imperiallines companies all use ships with the same specifications: 2000 dT jump-2 capable freighters. The ships differ in superficial details from company to company, but this is purely cosmetic and it's very easy to turn one design into another. Each company also secretly employs a version of their ship design which is capable of Jump-6. The short-legged ships handle the cargo hauling; the fast ships do the actual courier work.
Supposedly, each Imperiallines facility has two ships assigned to it, and they regularly return to that base. Actually, each facility has one jump-2 ship which works out of the base; the jump-6 ships have no permanent home, and instead move from place to place as their missions call for it.
The company in the Spinward Marches was originally supposed to be named Spinward Freighters, but for reasons that has never been made clear, the agent who handled the transaction used 'Imperiallines' instead. Rather than draw attention to the setup by changing the name, the Director decided that the name would work as a double bluff and left it alone.